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1.
An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad 18O(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted 18O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also 18O depleted. l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock 18O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential 18O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic 18O(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have 18O(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower 18O(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. * Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada  相似文献   

2.
The Ni-S System and Related Minerals   总被引：1，自引：0，他引：1  
The system Ni-S has been studied systematically from 200? to1, 030? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi3S2 (and a high temperature, non-quenchable Ni3?S2 phase),Ni7S6, Ni1–S4 Ni3S4, and NiS2. The geologic occurrenceof the minerals heazlewoodite (Ni2S2), millerite (ßSNi1-2S),polydymite (Ni3S4), and vaesite (NiS2) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556? C. This high-temperature phase(Ni3 has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600? C, and melts incongruently at 806??3? C.The ßNi7S6 phase inverts to Ni78 at 397? C6 when inequilibrium with Ni3S2, and at 400? C when in equilibrium withNiS. Crystals of Ni7S6 break down to Ni3-S2+NiS at 573??3?C.The low-temperature form of Ni1-S1 corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379??3?C, whereas Ni1-S with the maximum nickel deficiency invertsat 282??5OC. The Ni1-alphS-NiS2 solvus was determined to 985??3?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600?C but breaks down to Ni2-S2 and Ni1-S below797?C, whereas Ni1-S with 38.2 wt per cent sulfur melts congruentlyat 992??3?C. Vaesite does not vary measurably from stoichiometricNiS2 composition, and melts congruently at 1.007?5?C. Polydymitebreaks down to aNi-S? vaesite at 356??3?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991?C and over a wide compositionalrange.  相似文献   

3.
A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H2O = mineral + H+, (sum of cations) + H2O = (sum of normative minerals of a rock)+ H+. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logPH2O = 0) and chemical activity of the metal ions ( log Mn+= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog H+ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.  相似文献   

4.
Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9–monthinterval in 1982– 83, some 305 106 m3 of medium–K,calc–alkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An60–75)and two–pyrozene bearing andesites with 58% SiO2 to finalplagioclase (An85–90), diopside, and olivine (Fo85–90)bearing primitive magnesium basalts with 47% SiO2 Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of high–Mg parental melt. Theeruptive volumes of 35 106 m3 andesite, 120 106 m3 maficandesite, and 150 106 m3 basalt are consistent with the ideathat the 1982– 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have 18O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with 18Oplag–pxvalues of + 0.4 to + 0.6o, indicating internal O–isotopeequiliburium at temperature of 1100–850 C. The magenesianbasalts have magmatic 18O/ 16O ratios similar to those of mid–oceanridge basalt, and the O–isotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by 18O–rich crust during differentiation.Andesites and transitional mafic and sites have a more variableO–isotope character, with laves and phenocrysts havingboth higher and lower 18O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 1982–83eruption.  相似文献   

5.
Pelitic and calcareous rocks in the Whetstone Lake area havean unusually wide range of chemical composition. Metamorphicreactions have been deduced that represent the observed ‘discontinuities’in compatible mineral assemblages, and by plotting the reactantand the product assemblage of each reaction on a map, metamorphicisograds have been delincated ‘from both sides’.For the pelitic rocks, successively higher-grade isograds arebased on the following reactions: (1)chlorite+muscovite+garnetstaurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz kyanite+biotite+water; (3) kyanitesillimanite; (4)staurolite+museovite+quartzsillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartzCa-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4)in such a manner as to indicate that the H2O/CO2 fugacity ratiowas significantly higher in the vicinity of a granite plutonthan in the metasedimentary rocks remote from the pluton. Chemicalanalyses of the coexisting minerals in reaction (5) indicatethat the real reaction may involve plagioclase, epidote, sphene,and Fe-Ti oxides as well.  相似文献   

6.
Re—Os and Sm—Nd isotopic data have been obtainedfor mafic and ultramafic cumulates from the 2700-Ma StillwaterComplex and associated fine-grained sills and dykes, so as tobetter constrain the geochemical characteristics of Stillwaterparental magmas and to trace the source(s) of the precious metalsthat have been concentrated in the J-M Reef, the major platinum-groupelement mineral deposit in the complex. Initial Os isotopiccompositions (187Os/188Os) for chromitites from the Ultramaficseries range from a radiogenic isotopic composition of 0.1321(Os = +21) for the platinum group element (PGE)-enriched B chromititeseam from the West Fork area to a near-chondritic isotopic compositionof 0.1069–0.1135 (Os=–2 to +4.1) for the PGE-poorG and H chromitite seams, respectively, near the middle of theUltramafic series. Osmium isotopic data for the PGE-rich B chromititeseam are generally isochronous with whole-rock and mineral datafor the J-M Reef (Os = + 12 to + 34). Re—Os isotopic datatherefore document a contrast between PGE-poor cumulates fromthe Ultramafic series and PGE-enriched cumulates from both theUltramafic series and the J-M Reef, suggesting that Os and probablythe other PGE were derived from at least two isotopically distinctsources. Moreover, these Re-Os isotopic characteristics correlatewith petrogenetic subdivisions of the Stillwater Complex basedon field mapping, petrology, REE geochemistry, and Sm—Ndisotope geochemistry. The data are best explained by mixingof two magma types, referred to as U-type and A-type magmas,with differing major element, trace element, and precious metalabundances and isotopic compositions. Although crustally contaminatedkomatiites can mimic the Os and Nd isotopic characteristicsof the U-type magma, the combination of low initial Os isotopicvalues (Os0) with low initial Nd isotopic values (Nd–1),high 207Pb/204Pb for a given 206Pb/204Pb (Wooden et al., 1991),and high (Ce/Yb)n ratios in U-type cumulates and fine-grainedsills and dykes is more consistent with the involvement of aRe-poor, but trace-element-enriched portion of the subcontinentallithospheric mantle in the petrogenesis of Stillwater U-typemagmas. However, the radiogenic initial Os isotopic compositionsof the J-M Reef and other portions of the intrusion with elevatedPGE concentrations suggest that A-type parental magmas incorporatedOs from radiogenic early Archaean crust. The relatively largerange in (Ce/Yb)n, Os, and Nd values suggests that mixing ofgeochemically distinct magmas may have been an important processthroughout the history of the Stillwater magma chamber. Magmamixing may then explain not only the PGE-enriched J-M Reef butalso the anomalous enrichment of the PGE in the B chromititeseam from the West Fork area and the variable values observedin other chromitite seams of the Ultramafic series. The intimateassociation of these magma types, derived from or modified inthe Archaean continental lithosphere, may then be crucial tothe formation of magmatic PGE mineral deposits.  相似文献   

7.
Alteration of a Komatiite Flow from Alexo, Ontario, Canada   总被引：5，自引：4，他引：1  
To investigate the ability of komatiites to preserve duringalteration a record of the geochemical and isotopic compositionof the Archean mantle, we studied the petrology and geochemistryof a komatiite flow from Alexo, Ontario. Although this flowis relatively well preserved, two main types of alteration arerecognized: (1) hydration of variable intensity throughout thewhole flow; (2) local rodingitization. In samples only slightlyaffected by hydration, the alteration was essentially isochemical;where more intense, hydration was accompanied by the mobilityof TiO2, Zr and the light rare-earth elements (LREE). The mobilityof these reputedly immobile trace elements is influenced bytexture and secondary mineral assemblages. Rodingitization wasaccompanied by still greater mobility. An Nd isochron with anage of 272698 Ma and Nd(T) =+ 2.5 is defined by wholerock samples.Because the spread of Sm-Nd ratios is far greater than can beexplained by magmatic processes, the age is interpreted as thatof the main alteration event during or soon after emplacement.Scattered initial Sr isotopic ratios and Nd (T) (+ 3.6 to +0.7)provide evidence for several younger events. The Nd (T) of magnaticclinopyroxene ( plus;3.8) is higher than that of mafic tofelsicrocks and pyroxenes from the Abibiti belt (Nd (T) +2.5). Thisresult implies: (1) the komatiites exchanged Nd with surroundingrocks during waterrock interaction; (2) the isotopic compositionof komatiites and their mantle source differed from that ofthe more common basalts of the Abitibi belt. KEY WORDS: alteration; Archean; Komatiite; trace elements; radiogenic and stable islotopes *Present address: Ore Genesis Research Group, VIEPS Department of Earth Science, Monash University, Clayton, Vic 3168, Australia. Telephone: (61) 3 9905-3881. Fax: (61) 3 9905–4903. e-mail: ylahaye{at}artemis.earth.monash.edu.au  相似文献   

8.
Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM2 at 1.6 Ga, M2 ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M2 grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:18O(carb) 14–20%; metapelites: 18O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(18O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M2 contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing XCO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology *Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au  相似文献   

9.
BAKER  A. J. 《Journal of Petrology》1990,31(1):243-260
Stable isotope compositions of Ivrea Zone marbles and associatedlithologies are in general heterogeneous. The oxygen isotopecomposition of quartz in pelites ranges from 18O +9 to + 17(SMOW) and does not vary systematically with metamorphic grade.Peridotites retain oxygen isotope signatures close to mantlevalues. Marble calcites vary in isotopic composition from 13C + 2(PDB),180 +24(SMOW)to 13C –6(PDB), 18O + 13 (SMOW).Depletions in 18O and 13C may be explained dominantly by interactionwith fluids derived from within the observed metasedimentarysequence during prograde metamorphism. 18O and 13C show gradients of greater than 5/m across marblemargins and within marbles. The preservation of such isotopicgradients is not consistent with the long-term presence of grain-boundary-scaleinterconnected fluid films in and around marbles. There is ageneral lowering of 18O within individual marble bodies althoughlarge carbon and oxygen isotopic gradients are present. Calcitein marbles may attain oxygen isotope equilibrium, but rarelycarbon isotope equilibrium, with surrounding metapelites. Infiltrationof marbles must involve a component of channelized fluid flow. The general lack of isotopic equilibration within the sequencerequires channelized fluid flow and limited fluid-rock ratios.Large pervasive mantle to crust fluid fluxes are not consistentwith the observations. *Present address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, England  相似文献   

10.
FERRY  JOHN M. 《Journal of Petrology》1988,29(6):1121-1159
Mineral reactions at the biotite isograd were investigated inpelitic schists, micaceous sandstones, micaceous limestones,and metaigneous rocks from three stratigraphic units over anarea of 10000 km2 in north-central New England. The biotiteisograd in north-central New England represents a metamorphicdecarbonation front that affected all major rock types in eachstratigraphic unit. Pressure at the isograd was near 3500 bat the northern end of the study area and near 5500 b in thesouth. Temperature was in the range 400–450?C. Equilibriummetamorphic fluids were approximately CO2-H2O mixtures withXCO2=0?04–0?07. Volumetric fluid]-rock ratios were calculatedfor more than 70 samples of all major rock types from each formationusing measured progress of the prograde reactions and the estimatedP-T-XCO2, conditions of metamorphism. Regardless of stratigraphicunit, limestones record low values of 0–0?2, pelites andmetaigneous rocks generally record high values of 1–3,and standstones record intermediate values of 0?2–1. With exception of the limestones, all samples from the biotitezone record fluid-rock ratios significantly greater than likelyrock porosity during metamorphism. The prograde decarbonationreactions therefore were driven by infiltration of rock by reactiveaqueous fluids. The observed correlations between fluid-rockratio and rock type indicate that significant permeability contrastsoccurred during low-grade metamorphsim with permeability increasingin the order: limestones<sandstones<pelites rocks. Asa corollary, reactive fluid flow must have been channelizedwith enhanced flow in pelites and metaigneous rocks relativeto sandstones and limestones. Results of this study in north-centralNew England taken together with studies of the biotite isogradin south-central Maine (Ferry, 1984, 1986a, 1988) demonstratethat low-grade metamorphism over much of the northern Appalachianorogen was infiltration-driven.  相似文献   

11.
Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al2O3/TiO2 ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial 187Os/188Os of 0·1131 ± 0·0006(2), Os(I) = 0·1 ± 0·5. These data, coupledwith an initial Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment  相似文献   

12.
A Stable Isotope Study of Anorogenic Magmatism in East Central Asia   总被引：8，自引：0，他引：8  
A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral 18O/16O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme18O depletio (18O feldspar<–12). This reflects thevery low 18O of the water involved in these systems, which probablyhad 18O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal 18O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the 18O/16Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing 18O/16O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic 18O value of plutonic igneous rocks.Most sphene and quartz 18O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in 18O/16O, indicating that, on a regionalscale, the magmatic 18O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower 18O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes 1Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK.  相似文献   

13.
This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have 18O values(6·1–8·4) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high 18O crustal material. In this regard, the 18O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower 18O values (average 6·3± 0·1) at given SiO2 contents relative to rockserupted from distal vents (>7·0 km; average 7·1± 0.1). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ± 0·2). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the 18O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high 18O continental crust (9·0) increasinglyhybridized by lower 18O (6·0) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source 18O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the18O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen  相似文献   

14.
Progress () of the infiltration-driven reaction, 4olivine +5CO2 + H2O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m3 volume of rock. Mineral and stableisotope compositions record that XCO2, 18Ofluid, and 13Cfluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress  相似文献   

15.
The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCN–YbCN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has 18O values of 7·8 to 8·6and 13C values of –3·9 to –4·6. 18Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in 18O betweencalculated syenitic melts and carbonatites, and the overlapin Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite  相似文献   

16.
Geochemical and 40 Ar—39 Ar studies of the Malaita OlderSeries and Sigana Basalts, which form the basement of Malaitaand the northern portion of Santa Isabel, confirm the existenceof Ontong Java Plateau (OJP) crust on these islands. Sr, Nd,and Pb isotopic ratios of Malaita Older Series and Sigana lavasfall within limited ranges [(87Sr/86Sr)T= 0.70369–0.70423,ENd(T)= + 3.7 to +6.0, and 206Pb/204Pb = 18.25–18.64]virtually indistinguishable from those found in the three OJPbasement drill sites as far as 1600 km away, indicating a uniformhotspot-like mantle source with a slight ‘Dupal’signature for the world's largest oceanic plateau. Three chemicaltypes of basalts are recognized, two of which are equivalentto two of the three types drilled on the plateau, and one withno counterpart, as yet, on the plateau; the chemical data indicateslightly different, but all high, degrees of melting and slightvariation in source composition. All but one of the 40Ar-39Arplateau ages determined for Malaita Older Series and SiganaBasalt lavas are identical to those found at the distant drillsites: 121.30.9 Ma and 92.01.6 Ma, suggesting that two short-lived,volumetrically important plateau-building episodes took place30 m.y. apart. Aside from OJP lavas, three isotopically distinctsuites of alkalic rocks are present. The Sigana Alkalic Suitein Santa Isabel has an 40 Ar-39 Ar age of 91.70.4 Ma, the sameas that of the younger OJP tholeiites, yet it displays a distinct’HIMU‘ -type isotopic signature [206Pb/204Pb 20.20,(87Sr/86Sr) T 0.7032, Nd(T) 4.4], possibly representing small-degreemelts of a minor, less refractory component in the OJP mantlesource region. The Younger Series in southern Malaita has an40Ar-39Ar age of 44 Ma and isotopic ratios [Nd(T)=-0.5 to +1.0,(87Sr/86Sr)T =0.70404–0.70433, 206Pb/204Pb = 18.57–18.92]partly overlapping those of the ‘PHEM’ end-memberpostulated for Samoa, and those of present-day Rarotonga lavas;one or both of these hotspots may have caused alkalic volcanismon the plateau when it passed over them at 44 Ma. The NorthMalaita Alkalic Suite in northernmost Malaita is probably ofsimilar age, but has isotopic ratios [(87Sr/86Sr) T 0.7037,Nd(T) +4.5, 206pb/204pb 18.8) resembling those of some OJP basementlavas; it may result from a small amount of melting of agedplateau lithosphere during the OJP's passage over these hotspots.Juxtaposed against OJP crust in Santa Isabel is an 62–46-Maophiolitic (sensu lato) assemblage. Isotopic and chemical datareveal Pacific-MORB-like, backarc-basin-like, and arc-like signaturesfor these rocks, and suggest that most formed in an arc—backarcsetting before the Late Tertiary collision of the OJP againstthe old North Solomon Trench. The situation in Santa Isabelappears to provide a modern-day analog for some Precambriangreenstone belts. KEY WORDS: oceanic plateaux; Ontong Java Plateau; Solomon Islands; Sr-Nd-Pb isotopes; age and petrogenesis *Corresponding author.  相似文献   

17.
A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New 40Ar/39Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO2 and Zr contents, and reinforced by rare earth element (REE),87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO2 and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO2 and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, 87Sr/86Sri 0·7035, Ndi 9). However, theyhave very high TiO2 (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO2–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke  相似文献   

18.
The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites. Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive Nd(T) values(+1.1 to +3.9). Granitoid initial 87Sr/86Sr and most 18 O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/). Restricted 207Pb/204Pbvariation is accompanied by significant dispersion of 206Pb/204Pband 208Pb/204Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negativeNd(T) and a large range in 207Pb/204Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland *Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca  相似文献   

19.
Metamorphic isograds and time-integrated fluid fluxes were mappedover the 1500 km2 exposure of the Waits River Formation, easternVermont, south of latitude 4430'N. Isograds based on the appearanceof oligoclase, biotite, and amphibole in metacarbonate rocksdefine elongated metamorphic highs centered on the axes of twolarge antiforms. The highest-grade isograd based on the appearanceof diopside is closely associated spatially with synmetamorphicgranitic plutons. Pressure, calculated from mineral equilibria,was fairly uniform in the area, 7 1.5 kb; calculated temperatureincreases from {small tilde} 480C at the lowest grades in thearea to {small tilde} 575C in the diopside zone. CalculatedXco2f equilibrium metamorphic fluid increases from <0-03at the lowest grades to 0.2 in the amphibole zone and decreasesto 0.07 in the diopside zone. Time-integrated fluid fluxesincrease with increasing metamorphic grade, with the followingmean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclasezone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105;diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates twolarge deep-seated ({small tilde} 25-km depth) regional metamorphichydrothermal systems, each centered on one of the major antiforms.Fluid flowed subhorizontally perpendicular to the axis of theantiforms from their low-temperature flanks to their hot axialregions and drove prograde decarbonation reactions as they went.Along the axes of the antiforms fluid flow was further focusedaround synmetamorphic granitic intrusions. In the hot axialregion fluid changed direction and flowed subvertically outof the metamorphic terrane, precipitating quartz veins. Estimatesof the total recharge, based on progress of prograde decarbonationreactions, nearly match estimates of the total discharge, basedon measured quartz vein abundance, (2-10) x 1012 cm3 fluid percm system measured parallel to the axes of the antiforms. Withinthe axial regions fluids had lower XCO2 and rocks record greatertime-integrated fluxes close to the intrusions than at positionsmore than {small tilde} 5 km from them. The differences in bothfluid composition and time-integrated flux can be explainedby mixing close to the intrusions of regional metamorphic fluidsof XCO2/ with fluids from another source with XCO2{small tilde}0 in the approximate volume ratio of 1:2.  相似文献   

20.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination  相似文献   

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